Centralizers are commonly used in oil and gas wellbore installations and generally serve to center a pipe or casing within the wellbore during run-in, installation and cementing procedures. Conventional centralizers are typically characterized by a pair of opposed stop collars or stop rings with a number of outwardly-bowed springs extending therebetween to contact the wellbore sidewalls and exert a centering force on the pipe or casing segment.
As drilling projects are pushed to greater well depths, increased drilling angles, and through a greater variety of formations, more challenging demands are placed on centralizers and other down-bore equipment. For example, deeper wells are requiring more stages and passage of centralizers through a greater range and number of corresponding restrictions.
Conventional bow springs can suffer from a number of disadvantages in such installations. For example, conventional centralizer bow spring profiles often present steep lead-in angles which can make them difficult to compress. As the bore restrictions become increasingly tighter, the starting or insertion force and running forces required to pass restrictions increase significantly. Additionally, compression of the bow springs through particularly tight restrictions can exceed the elastic range of the material, leading to deformation of bow springs, compromising the restoring and centering ability thereafter.
U.S. Pat. Nos. 3,575,239 and 8,196,670 propose to reduce centralizer starting or running forces via alternating longitudinal offsetting of the bow springs. For example, the '239 patent proposes offsetting bow springs that are “symmetrical from end to end” by varying the bow spring attachment points, e.g., welds, on the stop collar. Similarly, the '670 patent proposes shifting symmetrical bow arch profiles towards one or the other of the stop collars via provision of a flat extension or “substantially axis parallel portion” at alternating ends of the bow spring profiles. Thus, both of these earlier proposed centralizers rely solely on longitudinal offsetting of otherwise symmetrical bow springs to reduce the required starting force, i.e., by reducing the number of bows simultaneously starting through a restriction. Such centralizers can still require abrupt starting forces, although incrementally divided somewhat by the longitudinal offsets.
Accordingly, improvements are sought in reducing and controlling starting or insertion forces and running forces and preserving centralizer integrity against the increasing demands of deep-well drilling.